1. Field of the Invention
The present invention relates to an output stage circuit, and more particularly, to an output stage circuit capable of eliminating the body effect and applied in a half supply voltage structure.
2. Description of the Prior Art
An operational amplifier (op-amp) is a basic circuit component, frequently used in analog integrated circuits. For reducing power consumption, the conventional operational amplifier circuit is utilized in a partition supply voltage structure. An illustration of this structure is shown in FIG. 1, which is a schematic diagram of a conventional operational amplifier circuit utilizing a partition supply voltage structure. The operational amplifier circuit 10 comprises an operational amplifier OP1 and an operational amplifier OP2. The operational amplifier OP1 receives a first voltage VDD through a first power terminal PW1 and receives a second voltage VDD_H through a second power terminal PW2. The operational amplifier OP2 receives the second voltage VDD_H through the second power terminal PW2 and is coupled to ground GND through a third power terminal PW3. In such a condition, if the second voltage VDD_H is half the first voltage VDD, the operational amplifiers OP1 and OP2 are utilized in a half supply voltage structure: the operational amplifier circuit 10 is a half supply voltage operational amplifier. Supply voltage of the operational amplifier OP1 is within a range from the voltage VDD to the voltage ½ VDD. Supply voltage of the operational amplifier OP2 is within a range from the voltage ½ VDD to ground. In such a condition, the output interval of the operational amplifier OP1 is within a range from the voltage VDD to the voltage ½ VDD and the output interval of the operational amplifier OP2 is within a range from the voltage ½ VDD to ground. As a result, the power consumption of the operational amplifier circuit 10 can be significantly reduced.
Although the above circuit utilizing the partition supply voltage structure may reduce power consumption, the operational amplifier circuit may work abnormally due to the body effect. Please refer to FIG. 2, which is a schematic diagram of the operational amplifier OP1 shown in FIG. 1. As shown in FIG. 2, the operational amplifier OP1 comprises an output stage circuit 202. The output stage circuit 202 consists of a transistor NOUT and a transistor POUT, wherein the transistor NOUT and the transistor POUT are in a cascaded formation. The base of the transistor NOUT is coupled to the lowest voltage of the operational amplifier circuit 10 (i.e. to ground) and the supply voltage interval is within a range from the voltage VDD to the voltage ½ VDD (i.e. the source voltage of the transistor NOUT is ½ VDD). In such a condition, the transistor NOUT has the body effect, such that the threshold voltage of the transistor NOUT increases. For the output stage circuit 202, which is utilized for providing a huge current to drive post-stage loading in the operational amplifier OP1, the transistor NB1 may be cut off when the gate voltage of the transistor NOUT is significantly increased due to the serious body effect. As a result, the output current of the output stage 202 may be limited to an extremely small current, such that the operational amplifier OP1 works abnormally.
Please refer to FIG. 3, which is a schematic diagram of the operational amplifier OP2 shown in FIG. 1. As shown in FIG. 3, the operational amplifier OP2 consists of a transistor NOUT and a transistor POUT, wherein the transistor NOUT and the transistor POUT are in a cascaded formation. The base of the transistor POUT is coupled to the highest voltage of the operational amplifier circuit 10 (i.e. to the voltage VDD). Since the supply voltage interval of the operational amplifier OP2 is within a range from the voltage ½ VDD to ground, the source voltage of the transistor POUT is ½ VDD. In such a condition, the threshold voltage of the transistor POUT is increased due to the body effect. The transistor PB1 may be cut off when the gate voltage of the transistor POUT decreases significantly due to the serious body effect, such that the operational amplifier OP2 works abnormally.
In the prior art, an independent P-well and independent N-well provided by special processes are used for eliminating the body effect generated when utilizing the above partition supply voltage structure. Utilizing these special processes, however, causes the manufacturing cost of the integrated circuit to be greatly increased, which is not ideal for the designer of the integrated circuit. How to eliminate the body effect generated by utilizing the partition supply voltage structure without using the special processes has therefore become a problem to be solved in the industry.